Explain the Acoelomates - Animals without a Body Cavity?
The simplest group of animals that has bilateral symmetry and a solid body (acoelomate) is the Platyhelminthes.
Phylum Platyhelminthes : These organisms are commonly known as the flatworms-"platy" of Greek derivation means "flat," and "helminth" means "worm." They are flattened dorsoventrally (top to bottom), and they have an anterior region that forms a distinct head. Flatworms range in size from a few millimeters to 20 meters in length. Although these animals have simple bodies, they do have several specialized organs.
Typically, members of this phylum have a gastrovascular cavity with one opening. Food is taken in through a muscular pharynx, and any undigested materials pass out the same way. Digestive enzymes break down the food, which can be distributed throughout the body by the branching intestine (gastrovascular cavity), since all of the body tissues are in close proximity to it. Parasitic forms of flatworms, such as tapeworms that live in the intestinal tract of their hosts, do not even need a digestive system! Because they live in an environment of digested food, all they need to do is absorb some of the food in which they are continually bathed.
Flatworms do not have specialized organ systems for circulation or respiration, since their tissues are in such close contact with the aquatic environment. Gas exchange and absorption can take place directly by diffusion. Flatworms do, however, have excretory cells called ciliated flame cells that help them maintain their salt balance. They also have a primitive ladder-like nervous system with a simple brain at the anterior end. Chemical sensors and a pair of light-sensitive eyespots and are also found in the head.
Free-living members of the Platyhelminthes move by gliding with cilia on their epidermal cells, or by using their muscles to undulate in the water.
Flatworms that are free living reproduce by sexual or asexual means. Some species are able to divide into two individuals by splitting in half or by fragmenting into several parts. In sexual reproduction, most free-living flatworms have both male and female reproductive organs, making them hermaphrodites. As such, they are able to undergo cross-fertilization, whereby two worms fertilize each other simultaneously. Clutches of fertilized eggs are laid and will hatch in about two weeks. Parasitic forms of flatworms often have very complex life cycles that may require one or more hosts in order to complete.
There are 4 classes of Platyhelminthes: Class Turbellaria, Class Trematoda, Class Monogenea, and Class Cestoda.